This disclosure relates to cardio-respiratory monitoring and analysis, and more particularly to methods for diagnosing sleep disorders. More specifically, this disclosure is aimed at detection of sleep apnea using the electrocardiogram. The invention can be embodied in a form suitable for use in a dedicated medical setting, or in the home.
Sleep apnea is a significant public health problem. Current estimates are that approximately 4% of the male middle-aged population, and 2% of the female middle-aged population suffer from sleep apnea. Patients suffering from sleep apnea are more prone to hypertension, heart disease, stroke, and irregular heart rhythms. Continued interruption of quality sleep is also associated with depression, irritability, loss of memory, lack of energy, and a higher risk of car and workplace accidents.
Current techniques for detection and diagnosis of sleep apnea rely upon hospital-based polysomnography. A polysomnogram simultaneously records multiple physiologic signals from the sleeping patient. A typical polysomnogram includes measurements of blood oxygen saturation level, blood pressure, electroencephalogram, electrocardiogram, electrooculogram, electromyogram, nasal and/or oral airflow chest effort, and abdominal effort. Typically, signals are recorded from a full night's sleep and then a diagnosis is reached following a clinical review of recorded signals. In some patients a second night's recording is required. Because of the number and variety of measurements made, this test can be uncomfortable for the patient and also has a relatively high cost. In general, it is only performed in a dedicated medical facility.
A variety of techniques have been proposed for simpler systems to detect sleep apnea. Acoustic screening devices have been proposed which detect loud snoring, or which detect long periods of silence which may indicate a dangerously long acute sleep apnea episode.
These are disclosed in U.S. Pat. No. 4,715,367. Other acoustic-based devices are disclosed in U.S. Pat. Nos. 5,797,852, 4,306,567, 4,129,125, and United Kingdom Patent Specification No. 2,214,302.
Detection systems using only measurements of respiratory effort or flow have also been disclosed. U.S. Pat. No. 6,062,216 discloses the use of a light beam to detect breathing motion. U.S. Pat. No. 6,142,950 uses an airflow sensor attached to the upper lip to detect inspiration and expiration airflow. WO 99/34864 discloses a nasal thermistor which responds to changes in nasal airflow and hence provides assessment of apnea.
A variety of detection systems have been disclosed which use combinations of measurements to detect sleep apnea. U.S. Pat. No. 6,091,973 uses measurements of skin conductance, heart rate, and blood oxygen saturation to detect arousals from apnea or hypopnea. U.S. Pat. No. 5,769,084 relies on processing of combinations of nasal air-flow, chest wall effort, oxygen saturation, heart rate, and heart activity in order to identify the onset and duration of breathing disorder. U.S. Pat. No. 5,765,563 discloses a system for using measurements of airflow, heart rate, and oxygen saturation for detection of apnea, hypopnea, and oxygen desaturation. U.S. Pat. No. 5,275,159 discloses a system which combines heart rate, respiratory and snoring sounds, oxygen saturation, and bodily position to detect apnea. U.S. Pat. No. 5,105,354 discloses a system for combining respiration and heart rate to detect sleep apnea in infants. U.S. Pat. No. 4,982,738 discloses a system which combines heart rate, respiratory and snoring sounds to detect apnea. U.S. Pat. No. 5,291,400 incorporates a system for the analysis of heart rate variability, but not in relation to the detection of sleep apnea.
Japanese Patent Specification No. JP 5,200,001 discloses a technique for measuring chest wall motion and hence detecting apnea. U.S. Pat. No. 5,891,023 discloses a technique for using desaturation and resaturation events in oxygen saturation.
Disadvantages of solutions based on the prior art include a high level of complexity due to the number of measurements required, and/or relatively low levels of accuracy in correctly diagnosing sleep apnea. The present invention seeks to overcome the aforementioned disadvantages associated with the prior art
Review of Electrocardiogram Terminology
The Electrocardiogram:
The heart is a muscular organ containing four chambers. The two smallest chambers are the left and right atria and the two largest chambers are left and right ventricles. The heart alternatively contracts and relaxes at the rate of approximately once per second as it pumps blood around the body. During this cycle (or beat) there are changes to the electrical charge surrounding the heart cells that result in potential gradients on the body surface. Any two electrodes placed on the body surface can measure these potential gradients. The electrocardiogram signal is a plot of these body surface potential differences against time. Thus, the electrocardiogram is a non-invasive technique for measuring the electrical activity (or cardiac potentials) of the heart.
Normal Electrocardiogram
The normal electrocardiogram has a number of characteristic patterns associated with each beat of the heart: the P wave, the QRS complex and the T wave. A number of measurements are routinely measured from the electrocardiogram relating to various inter- and intra-beat characteristics. An example of an inter-beat measurement is the time duration between each R wave peak of the QRS complex, referred to herein as the RR interval. Examples of intra-beat measurements include PR and QT intervals. A typical electrocardiogram signal obtained from a standard lead configuration is shown in FIG. 1 and consists of the three standard waveform components. The PR interval and the QT interval are identified on FIG. 1 as well as the PR segment and the ST segment. The letters do not have physiologic significance but the corresponding waves do as they relate to the electrical activity in specific regions of the heart.
P Wave:
During a beat of the heart the first event normally visible on the electrocardiogram is the P wave. The P wave occurs as a result of the electrical activity associated with the contraction of the two atria. In some electrocardiograms the P wave may not be visible. The normal duration of the P wave is 0 (no visible P wave) to 100 ms, measured from the onset to the offset of the P wave.
QRS Complex:
The next event apparent on the electrocardiogram is the QRS complex, which results from the electrical activity associated with contraction of the two ventricles. A normal QRS complex is generally comprised of a Q wave, an R wave and an S wave. Every positive deflection in this complex is called an R wave. The first negative deflection prior to the R wave is termed a Q wave and the first negative deflection following the R wave is called an S wave. Second and third positive deflections are possible and are called an R′ wave and an R″ wave respectively. The initial part of the QRS complex is related to the activity of both ventricles and the latter part is principally the left ventricle. The QRS complex is a much larger signal than the P wave for two reasons. Firstly, the ventricles are closer to the chest surface than the atria and secondly the ventricles contain much more tissue than the atria.
The QRS duration is measured from the start of the Q wave to the end of the S wave. It represents the amount of time needed for ventricular depolarization and its normal duration is 50-100 ms.
T Wave:
The last major event of the electrocardiogram it the T wave and it corresponds to the electrical activity associated with the ventricles relaxing. The normal T wave duration is typically between 100-250 ms. The atria also have a relaxation phase. This is not visible on the electrocardiogram as it occurs at the same time as the much larger QRS complex.